[Btrfs-devel] Re: BTRFS partition usage...

[David Miller]

From: David Miller <davem at davemloft.net>
Date: Mon, 11 Feb 2008 23:21:39 -0800 (PST)

> Filesystems like ext2 put their superblock 1 block into the partition
> in order to avoid overwriting disk labels and other uglies.  UFS does
> this too, as do several others.  One of the few exceptions I've been
> able to find is XFS.
> 
> This is a real issue on sparc where the default sun disk labels
> created use an initial partition where block zero aliases the disk
> label.  It took me a few iterations before I figured out why every
> btrfs make would zero out my disk label :-/

Actually it seems this is only a problem with mkfs.btrfs, it clears
out the first 64 4K chunks of the disk for whatever reason.

The following patch cures the disk label spamming problem for me:

--- vanilla/btrfs-progs-0.12/mkfs.c	2008-02-06 08:37:45.000000000 -0800
+++ btrfs-progs-0.12/mkfs.c	2008-02-12 00:07:43.000000000 -0800
@@ -210,7 +210,8 @@ int main(int ac, char **av)
 		exit(1);
 	}
 	memset(buf, 0, sectorsize);
-	for(i = 0; i < 64; i++) {
+	lseek(fd, BTRFS_SUPER_INFO_OFFSET, SEEK_SET);
+	for(i = BTRFS_SUPER_INFO_OFFSET / sectorsize; i < 64; i++) {
 		ret = write(fd, buf, sectorsize);
 		if (ret != sectorsize) {
 			fprintf(stderr, "unable to zero fill device\n");